The sharpness of the cutting edge of a knife blade or similar cutting tool is an important characteristic for both domestic knives and industrial knives, as well as for cutting tools in general.
It has long been known that the hardness of a blade material is an important contributor to the ability of a cutting edge of the blade to retain sharpness, as a cutting edge made of softer materials quickly becomes blunt. On the other hand a knife blade is often made as a thin strip or as a sheet, and its edge must have some flexibility so as to avoid brittle fracture or chipping when used. The two characteristics of hardness and flexibility or toughness often contradict with each other as most hard materials are typically brittle and easy to fracture.
Historically various techniques including quenching, heat treatment or alloying have been used to achieve the best combination of these two characteristics.
U.S. Pat. No. 6,105,261 describes a self-sharpening blade having a first, harder layer with relatively high wear resistance that substantially defines a cutting edge, and a second, softer layer of material with lower wear resistance, located on one side of the first layer. The thickness of the harder layer is between 0.3 microns and 1.5 mm. The examples given in this US patent include knife blades produced by metalworking or mechanical processing such as rolling several sheets of steel, hot pressing and sintering powders containing diamond and other hard materials, as well as coating deposition on plastics. The mechanical processing typically results in the production of a relatively thick layer of hard material, and does not enable a good blade sharpness to be achieved.
Attempts have been made to produce a knife blade with a hard coating. U.S. Pat. No. 6,109,138 describes a knife blade with one side of its edge coated with a particulate material in a matrix. It is stated that the matrix is softer than the particulate material, and the coating is such that a considerable number of the particulates project from the matrix thereby defining a cutting tip on the blade edge. This knife blade has enhanced edge retention characteristics and finds practical applications, for example, in domestic kitchen knives. However, knives with this type of coating have a number of disadvantages that limit their applications. The coating process does not allow a thin coating to be produced—the coating thickness is typically 25–30 microns. The coating consists of randomly distributed hard particles in a substantially softer metal matrix, and this coating structure does not therefore serve to form a straight self-sharpening edge within the thickness of the coating layer. This sets a limitation on the sharpness that can be achieved with a blade having such a thick hard coating layer. Furthermore, the cutting edge formed by discrete particles of hard material projecting from a matrix does not provide a smooth cutting action, but instead acts by tensile tearing of the material being cut. This typically requires a higher force to be applied to the cutting edge as compared to a purely compressive cutting action of a scalpel, for example. The coatings are normally used in an “as-deposited” condition; in other words, there is no additional or post-machining performed on the coating itself, which typically has a rough morphology. This surface roughness and resulting increased friction between the coating and the material being cut further contribute to impede the cutting action. Accordingly, cutting tools provided with this type of coating are restricted in their application due both to limited sharpness and rough surface morphology (leading to tearing rather than cutting).
Various attempts to make blades with hard coatings consisting of tungsten carbide particles in cobalt or another soft metal matrix have shown that a so-called “self-sharpening effect” depends strongly on the coating structure and properties. For example, the High Velocity Oxygen Fuel (HVOF) process for deposition of a tungsten carbide in cobalt matrix coating does provide the self-sharpening effect and is used in practice. By way of contrast, a similar coating process known as plasma spraying, when used to deposit a WC/Co coating, does not achieve the self-sharpening effect. Although both HVOF and plasma sprayed coatings consist of tungsten carbide particles in a cobalt matrix, and are produced by similar methods of spraying, the difference in their performance to produce cutting tools demonstrates that it is not easy or obvious to achieve the self-sharpening effect. Indeed, producing coatings that provide the self-sharpening effect depends strongly on the coating characteristics such as hardness, porosity and microstructure, and requires extensive experimentation and analysis.
EP 0 567 300 describes a hard coating having a columnar crystal structure that extends away from a surface of a blank and to an outer face of the coating. However, the mechanism of wear and fracture in the columnar-structured coating does not provide an optimal structure for edge sharpness. The columnar coating wears by fracture of the microcrystalline columns and their groups, and does not allow sharpening within the coating layer. As a result, the edge sharpness is defined by the thickness of the coating.
These techniques, although enhancing the edge retention characteristics of a blade, do not generally enable a smooth and sharp scalpel-like blade to be formed. This is particularly important when the blade is used to cut thin paper (such as tissue) and similar materials that can easily be ripped or tom by an uneven edge.
U.S. Pat. No. 5,799,549 describes razor blades with both sides coated with an amorphous diamond coating having a thickness of at least 400 angstroms, typically about 2000 angstroms. This coating imparts stiffness and rigidity to a thin blade. However, the coating, which has a sub-micron thickness (400 angstroms is equal to 0.04 microns, 2000 angstroms is equal to 0.2 microns) and is formed on both sides of the blade, does not provide for a self-sharpening effect as the blade is used.
EP 0 386 658 and U.S. Pat. No. 4,945,640 describe a wear-resistant coating for sharp-edged tools and a method for its production. The coating is deposited by the method of chemical vapour deposition (CVD), has thickness from 2 to 5 microns and consists of a mixture of free tungsten with W2C or W3C, or a mixture of free tungsten with both W2C and W3C. In all variants of this coating there is an admixture of relatively soft metallic tungsten, as a result these coatings typically have moderate hardness, substantially lower than the hardness of pure tungsten carbides. Methods of depositing these coatings are further described in detail in EP 0 329 085, EP 0 305 917, U.S. Pat. Nos. 4,910,091 and 5,262,202. The coating is produced from a gaseous mixture of tungsten hexafluoride, dimethylether (DME), hydrogen and argon. In this process, low-volatility tungsten oxyfluorides are formed due to the reaction between WF6 and oxygen-containing DME. The tungsten oxy-fluorides are difficult to reduce with hydrogen and are buried in the coating layer. This requires an additional heat treatment of the coating described in U.S. Pat. No. 5,262,202 to improve the coating characteristics. The coatings described in these publications have relatively low hardness (below 3000 Hv, typically 2300 Hv), non-uniform structure, and as a result do not enable self-sharpening to be achieved. As described in U.S. Pat. No. 4,945,640 and EP 0 386 658, this coating improves the erosion and abrasion resistance of sharp-edged tools, but does not provide the self-sharpening effect. Without the self-sharpening effect, the hard coating provides only limited improvement in the retention of the sharpness of the cutting tool edge.
Coatings that reduce friction between a cutting blade and a material being cut help to improve the cutting action, and to enable a material to be cut with a lower amount of energy. This has been demonstrated for example with razor blades coated with thin layer of PTFE, which is known for its low friction properties. Although the PTFE coating does not change the razor blade sharpness, the blade can be moved with lower force and thus provides a perception of improved cutting action. Soft PTFE coatings are useful for gentle cutting applications such as shaving hair with a razor blade, but would not survive the more demanding cutting environment faced by machine knives, for example cutting paper, plastics, food products etc. In these conditions, a soft PTFE coating will be quickly abraded and worn away. The cutting action of a machine knife would benefit from a durable coating with low friction that is able to resist wear and abrasion.
The surface roughness of a cutting edge bevel, and in particular the surface roughness of a coating on a cutting edge, also has an effect on the cutting action. A rougher bevel surface often forms a rougher cutting edge with small serrations that contribute to cutting by a tensile tearing action. As compared to the purely compressive cutting action of a smooth scalpel blade, for example, a rough serrated knife would require higher force and higher energy for cutting. Serrated knives are considered as longer lasting than knives with a smooth cutting edge, although they have an inferior cutting action, especially when cutting delicate materials.